Follow-up control



March 16,1948. s. GODET 2,437,951

FOLLOW-UP CONTROL Filed aaron 1, 1945 Inventor-z Sidney Godet by /W'jfw/M His Attorney.

Patented Mar. 16, 1948 FOLLOW-U1 CONTROL Sidney Godet, Albany, N.

Electric Company,

Y., asslgnor to General i a corporation of New York Application March 1, 1943, Serial No. 477,508 6 Claims. (Cl. S18- 30) This invention relates to the control of electric motors, more particularly to follow-up control, and it has for an object the provision of a simple, reliable, inexpensive and improved control of this character.

A' further object of the invention is the provision of a follow-up control in which the followup motor is supplied from an electric valve type amplier with an alternating voltage which is an amplified replica of a periodically varying control voltage, the magnitude of which is proportional to the error of the system.

A still further object of the invention is the provision of stabilizing means for the system which superimposes on the signal voltage supplied to the amplifier a periodically varying voltage which is directly derived from the error voltage and is of such phase and magnitude as substantially to eliminate hunting of the system.

In carrying the invention into eiect in one form thereof, means are provided for producing a periodically varying control voltage of variable magnitude. An electric valve type amplifier serves to supply to the follow-up motor an am plied replica of this control voltage. This amplier has an output circuit which is connected to the follow-up motor and an input circuit which has connections to the control voltage producing means. For stabilizing the operation of the system, an A. C. resistance-reactance bridge type network is provided. The electrical constants of this bridge are so designed that the modulation envelope of the output voltage is the first derivative of the modulation envelope of its input voltage. The error voltage of the system is supplied to the input terminals of this bridge network and the output voltage of the network is supplied to the input circuit of the electric valve type ampliner as an anti-hunt or stabilizing voltage. The signal voltage which is supplied to the input circuit oi the electric valve type amplifier is derived from the voltage across one arm of the bridge.

For a better and more complete understanding of the invention, reference should now be had to the following specification and to the accompanying drawing in which Fig. 1 is a simple diagrammatical illustration of an embodiment of the invention; and Figs. 2, 3, 4, and are charts of electrical characteristics of the resistance-reactance bridge networkvwhich serves to facilitate an understanding of the operation of the invention.

Referring now to the drawing, an object I0 is to be driven into positional agreement with a 2 pilot device or director II by suitable driving means, such, for example, as represented by the universal electric motor I2 to the drive shaft of which the object I ii is connected by means of suitable reduction gearing (not shown). An alternating voltage is supplied to the armature of motor I2 by means of a suitable electric valve type amplifier I3 to the output terminals of which the armature of the motel' I2 is connected 'by' means of conductors I li.

Although the amplifier I3 may be of any suitable type, it is preferably a class A-Bz multistage amplifier having a push-pull output stage driven by a class A single-ended stage. Ampliers of this character are available on the market and since the details of the amplifier per se constitute no part of this invention, the amplifier is illustrated conventionally in the drawing. Direct current power is supplied to the power input terminals of the amplifier from a suitable source, which is represented in the drawing by the two supply lines l5 and i6. This direct current power is supplied to the anodes and cathodes oi the electric valve comprising the successive stages of the amplier.

The first or input stage of the amplifier I3 comprises a single triode vacuum valve Il which, for

convenience in explanation, is illustrated outside the rectangle which conventionally represents the entire amplifier i3.

This amplifier supplies to the armature ci the universal motor I2 a faithful, but greatly amplied, replica of any signal voltage which is impressed on the input circuit -of the .amp1ier, i. e., across the grid lla and the cathode Ilo of the irst stage triode valve Ii.

For the purpose of supplying a voltage to the input circuit of valve Il which is proportional to the system error, i. e., the positional disagreement of the driven object ill and pilot device il, the input circuit of valve il is connected to a source of alternating voltage IS through a transformer i9 having a center tap secondary winding and rotary induction apparatus illustrated as comprising a rotary induction device 20, referred to as the transmitter, and a somewhat similar rotaryl induction device 2i, rei'erred to as the receiver regulator. The rotary induction device 2@ comprises a rotor member 20a provided with a single phase primary winding (not shown) and a stator member 20h provided with a distributed three-element winding (not shown) which is physically similar to the polyphase Winding of an ordinary wound rotor induction motor. The stator and rotor windings `driven object I by means v resistor 23 and adjustable resistor are arranged in inductive relationship with each and the terminals of its stator winding arev connected to the terminals of the stator winding of the transmitter by means oi conductors 22 so that the voltages induced in the stator winding or the transmitter cause currents to now in the stator winding of the receiver regulator thereby to produce a magnetic field similar to the magnetic eld produced by the current owing in the rotor winding of the transmitter.

The rotor of the transmitter 2B is mechanically connected through suitable gearing (not shown) to the movable element of the pilot device il. The ratio of this gearing between the pilot device and the rotor of the transmitter may be any suitable value, but in the present embodiment ci the invention it is assumed to nel 1:1. The rotor of the receiver regulator 2| is connected either to the shaft of the motor |2 or to the shaft of the .ci suitable gearing (not shown) having the same ratio as the gearing between the pilot device and the transmitter.

The initial relative arrangement of the rotors of the transmitter 2B and receiver regulator 2| is such that when the system is in correspondence, the axis oi the rotor windingof the receiver 2| is at right angles to the axis of the magnetic field produced by the current flowing in the stator winding so that the voltage induced in the rotor winding is zero. Rotation of the rotor member of the transmitter causes a voltage to be induced in the rotor winding of the receiver owing to the shift in the position of the axis of the magnetic eld of the receiver relative to the axis of the coil oi the rotor member, and the magnitude of this induced voltage depends upon the relationship of the axis of this winding to the axis of the magnetic eld, e. g., when the axes of the magnetic ileld and of the rotor winding are parallel, the induced voltage is maximum, whereas when these axes are at right angles with each other, the induced voltage is zero. Thus, rotation of the rotor of the transmitter or of the receiver regulator will vary the magnitude of the voltage which is supplied through the transformer I9 tothe input circuit of the amplier which, in turn, will result in variation of the current owing in the anode-cathode circuits of the valves comprising the successive stages of the amplifier.

For the purpose an anti-hunting network is provided in the connections between the receiver regulator and the input stage of the amplier. This network is an A. C. resistance-reactance bridge type network of which the two halves |9a and |9b of the secondary winding of the transformer 2| constitute the ratio arms. The third arm of the bridge comprises permanent 24 connected in series relationship with each other and capacitor 25 connected in parallel with these resistors. The fourth arm of the bridge comprises the adjustable resistor 26, xed resistor 21 and capacitor 28 connected in series relationship with each other. Thus, the center tap |9c of the transformer secondary winding and the common terminal 29 oi the adjustable resistors 24 and 2B constitute the output terminals oi the bridge.

A voltage divider comprising series connected of preventing hunting in the loperati-on of the system,

nai'voltage is derived from secondary winding of the transformer and is supadjustable resistor 30 and fixed resistor 3| coni' of the secondary windnected across the h alf |9a ing. The slider 3Go of this voltage divider is connected to thecathode i'lb of the ilrst stage valve Il of the amplier and the 'terminal 29 is connected to the grid Ailu oi valve l1.

4 The bridge is balanced for the frequency of the source IB by adjustment oi the variable elements of the third and fourth arms of the bridge. When the bridge is balanced, the voltage at the output terminals lBc and 29 is zero and remains zero for any constant magnitude of the effective value of the input voltage.

from the voltage divider comprising resistors 30 1i the slider is'moved to the zero vposithe only and 3|. tion, i, e., to the top of resistor 3U, then voltage supplied to il is the anti-hunt voltage which appears across the center tapic and terminal. lBy moving the sliderdownwardirom the zero position a sigthe half iSd of the plied to the input circuit of the amplier. The magnitude of this signal voltage is increased in proportion to the movement of the slider away from the zero position.

- For the purpose of adjusting the phase 4of the current supplied tothe armature |2a of motor l2 relative to the curren .suppliedto the field winding |2b, a suitable phase shifting network comprising a choke coil 32, capacitors 33 and 3d and an adjustable potentiometer 35 is provided. This phase shifting network is connected across the cathode follower resistance 36 in the cathode circuit of valve I1 and the input circuit of the second stage of the amplier is connected across the potentiometer 85. By varying the potentiometer 35, the phase o f the output current of the amplier can be adj-usted through a substantial range oi values.

With the foregoing understanding of the elements and their organization, the operationof the system itself will readily be understood from the following description.

When the system is at rest, with the driven object i0 in positional agreement with the pilot device il, the system error is zero and zero voltageis induced in the rotor windings of the receiver regulator. Consequently, the input voltage of the anti-hunt network and the output both zero. This condition is represented in Fig. 2 in which the straight line curve 31 of zero magnituderepresents the input voltage to the anti-hunt bridge network and the straight line curve 38 of zero magnitude represents the output or anti-hunt voltage. Since the error voltage, i. e., the voltage supplied to the transformer I9, is zero, the signal voltage which appears across the midtap and slider 30a is also aero and consequently the amplier supplies zero output current to the armature of the follow-up motor i2 and the motor is coni' sequently at rest.

i. e., the voltageinduced in the secondary windings |911, ISb ofthe trans-- the input circuit of the'valve f Rotation of the pilot device I I in a clockwise direction produces a corresponding rotation of the axis of the magnetic neld of the receiver regulator 2| with the result that an error voltage is supplied to the primary winding of transformer I9 which is proportional to the positional disagreement of the pilot device and driven object. This error voltage induces a voltage in the secondary windings I9a and I9b which is represented in Fig. 3 by the sinusoidal curve 39. This voltage isl the input voltage of the anti-hunt bridge network and is accordingly denoted input voltage.

A signal voltage appears between the center tap I9c and slider 30a which is a replica of input voltage 39 and in phase therewith. This signal voltage is supplied tothe input circuit of the amplifier and an amplied replica oi this signal voltage is supplied to the armature I2a of followup motor I2. As a result, a. current is caused to now in the armature circuit of the follow-up motor which current is in phase with the field current. The motor is thus energizedfor rotation in a ydirection to drive the driven object IB in a clockwise directioninto positional agreement with the pilot device II. If the rotation of the pilot device is in a counterclockwise direction, thephase of the error voltage is reversed and the current supplied to the armature of the follow-up motor is 180 out of phase with the field current. As a result, the motor drives the driven object I Il in a counterclockwise direction towardv a position of correspondence with the pilot device.

As the magnitude of the error increases, the magnitude of the input voltage increases correspondingly. The rate of this increase is represented as being constant and this condition is indicated in Fig. 3 by the linear modulation envelope 40 of constant slope. Since the modulation envelope of the input voltage 39 is a straight line of constant slope and since the modulation envelope of the output or anti-hunt voltage appearing across midtap I9c and terminal 29 is the rst derivative of the modulation envelope of the input voltage, the output voltage must be a periodically varying voltage of constant magnitude as represented by the sinusoidal curve 4I of constant amplitudev having a modulation envelope 42 of zero slope. This output voltage is supplied to the input circuit of the ampliler as an anti-hunt voltage. It is in phase with the input voltage and, therefore, in phase with the signal voltage which, as pointed out vin the foregoing, is in phase with the input voltage. Thus, under conditions of increasing error in the system, the anti-hunt voltage adds to the signal voltage thereby further to increase the current supplied to the follow-up motor. As a. result, the'speed of the motor is increased and the error is decreased to a value at which a balanced condition of the system is established. 'This balanced condition exists when the error is reduced to such magnitude that the current supplied by the amplier to the follow-up motor causes the latter to drive the driven object lil at a speed equal to the speed of the pilot device.

The error remains constant at this value as long as the speed of the pilot device remains constant and consequently the input voltage to the anti-hunt bridge network remains constant, as indicated in Fig. 4 in which the input voltage is represented by the sinusoidal curve 43 of constant amplitude. Since the modulation envelope represented by the. straight line 44 has zero slope,

45 of zero magnitude.

yline curve 49.

the magnitude ofthe modulation envelope oi the output voltage, which is the iirst derivative of the modulation of the input voltage, is zero as represented in Fig. 4 by the straight line ,curve The envelope being zero, the voltage itself must also be zero. Consequently, when the error is constant the antihunt voltage is zero and the speed of the motor is controlled solely by the signal voltage.

The condition of decreasing error is illustrated in Fig. 5 in which the error or input voltage is represented by the sinusoidal curve 4B of decreasing amplitude and having a modulation envelope 41 of constant slope. The decreasing value of the input voltage produces an output or antihunt voltage which is represented by the sinusoidal curve 48. Since the modulation envelope of the output or anti-hunt voltage is the rst derivative of the modulation envelope of the input voltage which has a constant slope, the modulation envelope of the anti-hunt voltage has zero slope as represented by the horizontal straight Consequently, the anti-hunt voltage has constant maximum value. Owing to the decreasing magnitude of the input voltage, the output voltage is out of phase with the input voltage and, therefore, also 180 out of phase with the signal voltage which is in phase with the input voltage.

Thus, when the error is decreasing, the anti' hunt voltage subtracts from the signal voltage in anticipation 0i the condition of correspondence in the system. As a result, the voltage supplied to the follow-up motor is reduced to zero before the driven object comes into correspondence with the pilot device. Thus, the tendency to overshoot and hunt about the position of correspondence is substantially eliminated. During a very rapid decrease of error, the anti-hunt voltage may be of suiicient magnitude to reverse the polarity of the voltage supplied to the follow-up motor and thus bring about a strong braking action of the fo1low-up motor so as to bring it to rest without overshooting.

Although, in accordance with the provisions of the patent statutes, this invention is described as embodied in concrete form and the principle thereof has been explained together with the best mode in which it is now contemplated applying that principle, it will be understood that the apparatus shown and described is merely illustrative and that the invention is not limited thereto since alterations and modiflcations will readily suggest themselves to persons skilled in the art without departing from the true spirit of this invention or from the scope of the annexed claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

i. A follow-up control cornprising'in combination, means for producing a periodically varying control voltage, electric valve means for amplifying said voltage, an electric motor supplied from said electric valve means, and stabilizing means comprising a resistance-reactance network included in the connections between said control voltage producing means and said electric valve means for supplying to said electric valve means a periodically 'varying voltage having the same periodicity as the periodicity of said control voltage and having a modulation envelope which is the first derivative oi the modulation envelope of said control voltage.

2. A follow-up control comprising in combination, a source of periodically varying control voltage. Aalu-electric motor, means for supplying to said motor an ampliile'd 'replica of said periodically varying voltage comprising electric valve apparatus provided'with a control grid and elecy trical connections from said source to'said grid.

and means for stabilizing Vthe operation of said motor with respect to said control voltage comprising a resistance-reactance bridge includedln said connections for superimposing on the voltage supplied to said grid a periodically varying voltage having the same periodicity as the periodicity of said control vvoltage and having a modu# lation envelope which is the rst derivative of the modulation envelope of said control voltage.

v 3. A follow-up control comprising in combinap tion, a source of periodically varying control Vvoltage having a modulation envelope of variable magnituderan electric motor, means lfor supplying to said motor an ampliiied replica. of said fperiodically varying control voltage comprising electric valve apparatus provided with an output circuit connectedto saidV motor and an input circuit having connections to said source of control voltage, and'stabiliz'lng means for supplying to said input circuit a periodically varying voltage having the same periodicity as the periodicity of said control voltage and havinga modulation envelope which is the first; derivative ofl the modulation envelope of said control voltage. comprising a bridge network having apairoi ratio arms, a third arm comprising resistance and reactance in parallel, a fourth arm comprising resistance and reactance in series, and potentiometer means included in said connections for causing to be supplied to said input `circuit a signal voltage derived from said control voltage.

4. A follow-up control comprising in combination. a source of periodically varying control voltage having a modulation envelope of varying magnitude, a universal electric motor, having an armature and a eld winding, means for supplying to the armature of said. motor an amplied replica of said control voltage comprising electric valve apparatus provided with an output circuit connected to the armature of said motor and with an input circuit having velectrical Ycormec=I tions `to said source of control voltage, stabilizing means for supplying tosaid input circuit a periodically varying voltage having the same periodicity as the periodicity oi said control voltagel and having a modulation envelopewhich is the first derivative of the modulation envelope of said control voltage, means ior'supplying to said eld winding a periodically varying voltage having the same periodicity as said .control voltage. and

d said control voltage.

means for adjusting the phase'ot the voltage supplied to said amature relatlveto lthe voltage supplied to said ileld winding.

5. -A follow-up control comprising in combina- :5 tionfa source of periodically varying control voltage having a modulationV envelope of varying magnitude, an electric motor, means for supplying to said motor an ampliiled replica. of said signal voltage comprising velectric valve apparatus provided with an voutput circuit connected to said motor and with an input circuit having electrical connections to said source. 'and stabilizing means for superimposing on the voltage supplied to said input circuit a periodically varying voltage having amodulation envelope thatis `the ilrst derivative ofthe modulation envelope of said control voltage comprising av bridge network included'in saidconnections having a pair of ratio arms, a third arm comprising a resistor and a capacitor in parallel and a fourtharm having a resistor and a capacitor in series. Y

(i.l A follow-up control comprising in combinatonfa Source o .periodically.varying control Vvoltage having a modulation envelope of varying magnitude. an electric motor, means for supplying to Vsaid motor an amplified replica 9i said 4signal'voltage comprising electric valve apparatus provided, with anontpt 'circuit connected to saidmotor and with an input circuit, and stabilizing means for supplying to said input circuit a lperiodically varying voltage having a modulation envelope vthat is the rst derivative of the modulation envelope of said control voltage comprising a bridge network having a pair of ratio arms,

a third arm comprising a capacitor and a resistor in parallel and a fourth arm comprising a capacitor land a resistor in series, electrical connections rom said source for applying said control voltage across said ratio arms, electrical connec- 40 tionsl from the junction point of said ratio arms and the junction point of said third and fourth arms vto said input circuit, and means for varying said ratio arms thereby to cause to be supplied to said input circuit a signal voltage derived from SIDNEY GODET.

REFERENCES CITED The Afollowing references are of record in the ille of this patent:

` UNITED STATES PATENTS Number Name Date 2,038,654 H1111 I Allg. 3, 1937 

